Balloon Building - MacNutt

Balloon Building with Phil MacNutt

Building N9208Z, A Tribute to the Ed Yost Balloon “Silver Fox”

This wound up being long, so long that I needed to do it in two parts, my apologies to all readers. In the geek-blog world, this is known as “tl/dr”, which is short for “too long, didn’t read”.

Several years ago I wanted to build a balloon that was some other design than a standard envelope (i.e. “Natural Shape”), and I just finished reading Double Eagle and it came up me; It’s a sphere on a cone, what could be simpler than that? Originally I planned to build a Double Eagle balloon replica, which is the same balloon as the Silver Fox (flown by Ed Yost), except the silver material on top extends below the equator. By the way, Ed Yost was key in developing the natural shape envelope back in the 50’s.

The more I thought about it, I figured a Silver Fox replica would be better, since this was the first transatlantic gas balloon that Ed built. You might think it strange to build a hot air balloon shaped after a gas balloon, but it really does not matter much, except my suspicion is that the lift is somewhat less than a standard balloon. The typical hot air balloon envelope has been designed to absolutely optimize lift, resulting in lower envelope temps for a given lift. The flat top on these envelopes is what does it. I was not too concerned about this bit of reduced lift considering that this thing is rated to lift 2,000 lbs at max temp.

I spent quite a while doing calculations for the design; material qty, lift, cost, etc, and it looked like a good project. Picking a size was probably the most frustrating part. I plan to use this envelope for another high altitude flight, so it had to be big enough for that, but no bigger, since I despise hauling around big envelopes, especially when you have Skinny on your crew, who has limited lifting capacity. So half the work on the balloon was first figuring up what size I needed for the altitiude flight. I got it down to a 95K, which is about 25K more than I wanted. I ran the numbers using the 95K size, and with lightweight material, it looked like a finished envelope weight of 150 was possible. I suspect this is still about 75 to 100 lbs less than a standard store bought monster of the same size. A side note here, when the balloon was finished, I put it on a scale and it came in at…155 lbs. It’s really cool when science works.

The real magic on this design came from my friend Jim Sippel, who does mechanical drawings (CAD), and machining of small parts for me. Jim is brilliant and the best CAD guy I have ever seen. He is easily 10 times faster than anybody else. Here’s how we did it. I took a picture of the balloon that is on a copy of a National Geographic and sat down with Jim in front of the computer. We took various measurements using a wooden ruler, and determining size based on the flag on the balloon, which I had determined was a 5’x8’. How did I know this? Well, I measured how many pixels Ed was tall, and figure he is about 5’7”, and compared this to the flag. Sure enough, it came out really close to 5x8. This all sounds pretty crude, doesn’t it?

Jim first drew a sphere and a cone, and connected them together making a balloon. He then used the measurements to make the shape correct. I told him I needed the volume to be 95K, and he went to work. Less than an hour later he had the balloon designed precisely the dimensions to give me 95K in volume. I told him then that the balloon would be 18 gores, which is actually 36 vertical slices (2 halves make one gore). Jim created gore seams and then did the real magic. He took one slice and translated the curved surface to a flat pattern that would be used for cutting out the fabric. I trust Jim, but was a little nervous about wasting 1,000 yards of fabric if he missed a decimal place or something.

So I built a model, 1/10th scale. It stood about 6 feet high, and came out looking precisely correct. I ordered fabric, a LOT of fabric. I think it was a bit over 1,000 yards, almost equal parts of white and black. As a functionalist, this balloon has one too many colors for me. One color flies the same as many, and is a lot easier to build!

We made patterns out of craft paper and called a bunch of people to come over one weekend and help cut. We could not have done this project without them. We had a lot of fun doing it, but it was a lot of work. Each gore slice (36 of them) was composed of 5 sections; three black on the bottom, and two white on the top. So we had 180 individual panels to make. And these were not short panels; a couple of them ran 40 feet long each. In my other balloons, I’ve just had one long gore to keep it simple, but that was not possible on this design. The total gore length of this balloon is just short of 100 feet, it is a very tall balloon. Stretched out with the basket connected results in a laid out length of probably 110 feet. Can’t even fit in a standard fiesta spot!

I dusted off the sewing machine and spent a couple of hours (at least) oiling it (over 25 locations) and the absolute critical task of timing the “hooks”. The hooks are on the bottom side and are what grab the thread from the needle and pass it through the bobbin thread. If you don’t sew, this is just a bunch of blah-blah-blah. I will tell you how important this task is; If the timing is off by 1/32 of an inch, the machine will flat out not sew. It will shred thread, cut thread, jam thread, essentially anything to make you angry and have a tantrum.

Anyway, I got her all dialed in and started the first task; building the parachute. Mine is 21 feet in diameter; only it wound up 23 feet in diameter because I screwed up on the seam allowance dimension. I ripped out all 18 panels, slit one inch off each panel, and started over again. Is this not a great hobby or what!

Now to start on the main balloon; take the five pieces that make a single slice and sew them together into one long slice, 95 feet long. Do this 36 times; which winds up being a bit short of 1,000 feet of sewing. Step two; take two of the long strips you just made and sew them into a single whole gore. Do this 18 times; about 1/3 of a mile of sewing. After each seam, go back and inspect the entire 95 feet of stitching. Fix problems. When you mention balloon building, most folks think this task, sewing the long pieces of fabric into a balloon, is the whole project. In reality, this task is by far, the simplest and least time consuming of the entire project. It is absolutely trivial compared to the rest of the work.

Now you start sewing the balloon together, taking two gores, sewing them together, add another gore, and continue until you have a mountain of fabric on the floor next to the machine. But to complicate this step is the fact that you have to lay down the load tape on all these seams (another 1/3 mile total sewing). How much load tape is in this balloon? The bill was over $700 just for the tape, about 1/3 the cost of all the fabric. Now, I buy military spec webbing, which is more expensive, but makes me feel good when I am up in the air.

Like all my balloons, I built this one in my living room. The grand piano in the corner did not help matters one bit, I can tell you that right now. (A quick note to say that just because I have a piano does not mean that I am rich like Lloyd Cates). I terribly underestimated the amount of fabric that I was going to have to move around in this small room, simply because of the sheer size of this thing. When I needed to stretch out a gore to deal with some misery, I had to put one end at the front door, go through the living room, the dining room, the hall, and wind up at the back door. This was HALF the gore. It was at this point I realized I had no choice but to move operations out to the driveway, which luckily is quite long, about 100 feet. For those of you, who have been to my house, imagine the fabric stretched out from the garage all the way out to the street.